A heater malfunction halted the countdown April 29, and the trouble was traced to a switch box in Endeavour's engine compartment. (NASA)

The next-to-last space shuttle flight has been delayed again, this time to at least the middle of May for extra electrical tests.

Mission managers decided Friday that shuttle Endeavour would blast off no earlier than May 16.

The space station delivery mission led by Rep. Gabrielle Giffords' astronaut husband has been on hold for a week.

A heater malfunction halted the countdown April 29, and the trouble was traced to a switch box in Endeavour's engine compartment. The box was removed, and this week engineers discovered a blown circuit inside.

NASA spokeswoman Candrea Thomas said testing will be conducted throughout the weekend to find out if the circuitry problem was in the old box or somewhere in the external wiring that's still in the shuttle. A new unit was installed Wednesday.

Commander Mark Kelly will lead Endeavour's six-man crew to the International Space Station. They will deliver a $2 billion particle physics detector along with station spare parts.

Giffords, Kelly's wife, was shot in the head four months ago during a political event in Tucson, her hometown. She is recuperating at a hospital in Houston, Kelly's home base.

The congresswoman's staff said she will return to Cape Canaveral for another launch attempt. The April 29 launch attempt also drew President Barack Obama and his family.

A launch on May 16 would be at 8:56 a.m. ET.

On Thursday, space station astronaut Catherine Coleman told The Associated Press she's disappointed by Endeavour's delay. She also worries about the work that will have to be shared if she and two other astronauts are gone by the time the shuttle arrives.

Coleman, an Italian and a Russian are scheduled to depart for home May 23, via a Russian Soyuz capsule, leaving three residents behind. Their station replacements won't be on board until June 9.

"They'll come when everything is right," she said of the shuttle crewmen. "It's the way it goes for shuttle launches."

Only one other shuttle mission remains, by Atlantis this summer. That liftoff is targeted for June 28, but could be pushed back by Endeavour's repeated delays.

NASA is under presidential direction to end the 30-year shuttle program as soon as possible and to focus on interplanetary travel. Private companies, meanwhile, are competing for the opportunity to carry out cargo and crew haul to and from the space station.